The present invention relates to a method for analyzing the concentration of at least one component in a gas mixture on the basis of the absorption of infrared radiation, in which method: a radiation coining from a radiation source is allowed to pass through a gas mixture contained in a measuring cell, the absorption of at least said gas component to be measured having an effect on emerging radiation; the radiation going in or coming out of the measuring cell is allowed to pass trough an optical band pass filter transmissive at least to a certain first wavelength band; and the intensity of such filtered radiation is measured with a detector, which in the direction of radiation is positioned downstream of said first band pass filter and the measuring cell. Particularly, the invention relates to a measuring method based on infrared absorption for determining the concentration of carbon dioxide from a gas mixture, which most often also contains at least one other poorly absorbing gas component to be measured. The invention relates also to an apparatus for implementing the method.
The most common method of measuring carbon dioxide for example from alveolar air or an exhaust gas is to employ a measuring method based on non-dispersive infrared absorption. Carbon dioxide absorbs effectively over a range at 4.26 .mu.m, and this is the range normally used since other gases do not generally have disturbing absorption there. If the concentration to be measured is about 10% by volume at normal pressure, a suitable absorbance in terms of measuring accuracy will be obtained over a measuring length, i.e. a distance traveled by radiation in a gas mixture, of 3-10 mm. If the carbon dioxide content is higher, the optimum measuring length will be even shorter. When, for some reason, it is necessary to use a longer measuring length, i.e. a thicker sample cell, the absorption will be so much deeper that measuring accuracy suffers. Such a condition arises if the same sample cell is to be used for measuring both a poorly absorbing gas component, e.g. alcohol, and carbon dioxide. This situation often leads to a compromise regarding the measuring length of a sample cell, whereby the absorbance of neither gas component is optimally selected. A very short sample cell is inconvenient for other technical reasons as well.
Several efforts to overcome this problem have been described in literature. The simplest approach is to use two separate series-connected sample cells having an unequal measuring length for carbon dioxide and poorly absorbing gas components, the short cell being used for measuring carbon dioxide and the long cell for measuring poorly absorbing components. In practice, however, such a solution is complicated and expensive, and in high-speed measuring there can be no certainty as to the duration and precision of a time lapse between various gas components.
The publication EP-309 666 discloses the use of a less powerful absorption range in the neighborhood of 2.7 .mu.m. In principle, this would enable the use of a longer sample cell for measuring carbon dioxide but, as pointed out in the cited publication, water vapor absorbs in a disturbing manner over this range. Water vapor is present in substantial amounts in both alveolar air and for example in exhaust gas and, thus, the accurate measuring of carbon dioxide also requires the measuring of water vapor or the limination of its effect by some other means.
The spectral band of carbon dioxide over the range of 4.26 .mu.m consists of a plurality of rotational lines. Near the beginning and end of the spectral band, the lines are less effectively absorbing, The restriction of measuring to cover just these weakly absorbing lines would in principle enable the measuring of carbon dioxide also by means of a sample cell having a long measuring length. However, the successful measuring requires the use of a highly narrow band and sensitive optical filter and, since the position of the passband of a filter in terms of its wavelength is very near the sharp edge of the spectral band, the produced signal will be highly temperature sensitive. Even a minor change in the temperature of any device component may offset the passband of the filter e.g. in the direction of the spectral band or beyond the sharp edge, at which the absorption of carbon dioxide increases dramatically.
The publication U.S. Pat. No. 5,429,805 discloses the use of an optical gas filter to limit away the most intensively absorbing spectral lines of a spectral band. The gas filter in series with a specimen cell contains the same or a similar gas as the gas to be measured, whereby the most intensively absorbing lines of this gas filter remove or reduce measuring radiation at said lines before the radiation reaches a detector. On the other hand, the more weakly absorbing Lines of the gas filter are only capable of removing very little radiation and, thus, the measuring can be effected over the wavelengths represented thereby, Hence, it is possible to employ a longer specimen or sample cell or to measure higher concentrations without developing deflecting non-linearity. The method is basically functional but the use of an optical gas filter is always inconvenient and expensive and, in addition, the leak hazard of a gas filter is also a considerable risk factor. It is also likely that the collision broadening caused by other components in a gas mixture to be analyzed has a disturbingly high impact upon the measuring signal of a component to be measured. The reason for this is that other components in a gas mixture to be analyzed may cause variable broadening of the absorption lines included in the spectral bands of a gas component to be measured while in a gas filter such broadening is not likely to occur at all, since the gas in a filter has a higher purity, or the broadening is at least constant since, in any case, the gas mixture in a filter remains unchanged. A consequence of this is that, as far as the absorption lines are concerned, the measuring result is affected more by the fringe than center sections of the lines and, in fact, the outcome may be that the measuring is more related to the collision broadening, i.e. the interaction of gas components, rather than to the concentration of a desired gas component.